The atomic force microscope (AFM) has matured and developed into an instrument for the routine inspection of the surface topography of both insulating and conducting samples at an atomic resolution. Recent efforts have revealed a much broader potential for the measurement of local mechanical and electrical surface properties such as the hardness, surface potential, surface charge, and capacitance of a sample.
Force modulation microscopy (FMM), in which a probe tip or sample is mechanically modulated in contact mode, typically by a piezoelectric transducer, has been used to measure the local mechanical elasticity of a surface. Electrostatic force microscopy (EFM), which operates in noncontact mode, has been developed to investigate the electrical properties of a sample such as the dielectric constant, surface charge, and surface potential. Scanning capacitance microscopy (SCM) has been used to measure the dopant profile on a semiconductor surface.
Unfortunately, the earlier FMM techniques using mechanical modulation measure only the mechanical properties of the sample, such as hardness and viscosity. Furthermore, the mechanical modulation excites other normal modes of the microscope system, which not only causes uncertainty and noise in the measurement but also limits the maximum modulation frequency to a level below the resonant frequency of the mechanical transducer and supporting structure.
Conventional EFM is implemented in a noncontact mode AFM, where the force gradient (which includes the electrostatic force gradient as well as the van der Waals force gradient) is used to control the tip-sample distance. Therefore, the probe of the conventional EFM fails to follow the true topography of a sample where a large electrostatic force gradient is present. For example, abrupt changes in the surface potential or surface charge on the sample surface produce large electrostatic force gradients. In such cases, the AFM probe does not follow the true topography but follows the constant force gradient contour.
Such errors in following topography cause successive errors in measuring the electrical properties of the sample. In addition, noncontact mode operation is complicated and its spatial resolution is significantly lower than in contact mode operation since the tip is separated, approximately 10 nm, from the sample.
Therefore, what is desired is a system and method which (1) performs both mechanical and electrical measurements, (2) does not require the probe tip or sample to be mechanically modulated, and (3) operates in contact mode.